Re: [PATCH] iommu/io-pgtable-arm: Drop DMA API usage for CMOs

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Jan 08, 2026 at 01:44:51PM +0000, Mostafa Saleh wrote:
> On Thu, Jan 8, 2026 at 1:27 PM Mostafa Saleh <smostafa@google.com> wrote:
> >
> > On Thu, Jan 8, 2026 at 12:59 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Thu, Jan 08, 2026 at 11:38:46AM +0000, Mostafa Saleh wrote:
> > > > Jason pointed out that the DMA-API calls are not really needed [2].
> > > >
> > > > Looking more into this. Initially, the io-pgtable API let drivers
> > > > do the CMOs using tlb::flush_pgtable() where drivers were using the
> > > > DMA API (map/unmap_single) only to do CMOs as the low-level cache
> > > > functions won’t be available for modules.
> > >
> > > This isn't what I ment at all, the iommu-pages.h could do the flush
> > > inside itself using the already existing
> > > iommu_pages_flush_incoherent() instead of open coding the dma api
> > > calls everwhere, and maybe that could directly call the arch function
> > > on arm (as x86 does) which would be easier to implement in pkvm's
> > > hypervisor.
> > >
> >
> > I see, so basically, we add the check for IOMMU_PAGES_USE_DMA_API in
> > iommu_pages_flush_incoherent() with the DMA-API stuff and call it from
> > the io-pgtable-arm instead.
> >
> 
> iommu_pages_flush_incoherent() is also used to flush non-cacheable
> pages in generic_pt from flush_writes_*(), so it might not be that
> simple; We might need to add a new variant.

There are no non-cachable pages in the iommu-pages system???

All memory comes from folios and is mapped to the CPU cachable.

The general API requires the user of iommu-pages to know if the
underlying HW is going to do non-coherent DMA, in this case it must
call iommu_pages_flush_incoherent() before the HW does DMA.

Effectively what I'm saying here is to convert the arm page table to
use iommu-pages and then put your pkvm abstraction at the iommu-pages
level instead of trying to hack up all these users.

You can also do the same conversion to the smmu driver to use
iommu-pages instead of DMA API, AMD and Intel are doing this already.

Jason